Chironomidae is an incredibly diverse family within the order Diptera, the true flies, which itself is quite diverse. There are estimated to be over a million species of dipterans. Diptera are "true flies", a group that are characterized by their single pair of wings (other winged insects have two pairs of wings). By convention, true flies are written so the word "flies" is separated from their descriptor whereas "flies" outside of this group - like mayflies and caddisflies - are written as a single word. Dipterans include house flies, horse flies, mosquitoes, crane flies, "gnats", and black flies, among many, many other flies.
Of the major aquatic insect groups that fly anglers are familiar with, we tend to be least familiar with midges. Most fly anglers associate midges with being small in size but that it not always the case. While stream anglers are very often fishing midges in sizes #20 and smaller; lake anglers are often fishing larval and pupal imitations in sizes #10 to #14. "Buzzers" - the name given to midges in the UK - are as important to anglers on lakes as mayflies and caddisflies are to stream anglers. Lake and spring pond anglers are much more familiar with these aquatic insects compared to stream anglers - of which I am almost exclusively one.
The non-fly angler probably knows these best as "lake flies" which can hatch in numbers that are hard to fathom. They make bridges slippery and shoreline areas nearly unusable for a few weeks in the summer. Their larvae are also the main forage of Lake Winnebago's famed Lake Sturgeon population.
Dipterans are holometabolous meaning that like caddisflies, and more famously lepidopterans (butterflies and moths), they undergo a complete metamorphosis which includes separate larval, pupal, and imago (adult) stages. I will readily admit I know less about Chironomidae than I do the mayflies (Ephemeroptera), caddis (Trichoptera), and even the stoneflies (Plecoptera). But I am totally fine with that. Imitations in a few colors - black, a couple of shades of tan and gray, and red and you are likely quite well equipped. This is not to downplay their importance, they are just really difficult to identify and color is typically the least important thing to imitate. And they tend to be important in each of their life stages.
Midge larvae are quite common and widespread. They are often tolerant to pollution - but not all species are so tolerant - this is part of what leads to how widespread they are. Part of the reason fly anglers do not know genus and species or even the families of the midges they are imitating is that they are difficult to tell apart, even by experts. Entomologists that specialize in midges identify them largely by mounting their mouth parts and genitalia on microscope slides. Not going that far? Yeah, me neither.
Midge larvae generally very simple "worms" and easy to imitate with simple flies. Though, if it is your thing, you can get MUCH more into midge imitations. Holbrook and Koch's book, Midge Magic, gets quite into pumping stomachs and matching the not yet hatched midges. It is a deep dive into midges and how to best imitate them. But for most of us, a simple Zebra Midge is probably the pattern we are most familiar with and are most likely to carry in our fly boxes. The simple pattern is all that is really necessary, at least in the smaller sizes - #18 and smaller - which most of the time we are fishing.
Midges are found throughout the stream but one place that they then to be most numerous are in the sediments and on the vegetation in pools. As mentioned above, they tend to be very common in lakes. Many lake anglers become much more educated about and dedicated to midge fishing. Midges are generally much more tolerant to low dissolved oxygen than other aquatic insects - probably one of the reasons they live in sediments and lakes. For the stream angler, a few colors will generally suffice unless you want to fall down a rabbit hole into "Midge Magic" territory. Midge Magic is a fantastic book for the midge enthusiast and the authors really get into specific - yet simple - imitations of midges - their larva in particular, through extensive stomach sampling of trout. Any midge larva you are going tie will be simple; there just simply is not much to a midge larva. For most of us, Zebra Midges, Brassies, and other simple larvae imitations will work quite well on the rare occasion we fish these flies.
Midges move as pupa from where they lived as a larvae to the surface of the water where they hatch into aerial adults. This "story" is similar to that of caddisflies and similar to caddisflies, pupa are important for the fly angler because of how vulnerable they are to fishes. Pupa can be imitated in deeper waters with beadhead flies and at or near the surface with lighter wire hooks and small additions that help them hold in the surface film.
A quick YouTube search for chironomid pupa returned a ton of results - and most of these fly patterns are really similar to one another. Most midge pupa look a lot like the simple larvae patterns with the addition of something white to imitate their gills. This something white may be a bead, a bit of marabou or CDC, a synthetic yarn, or other material. Many of the buzzer patterns get just a little more complicated by adding some emerging wing cases as well. But none of these patterns are overly complex - they basically add a step or two to better imitate the pupal stage and the transition to adulthood. The other common imitation of pupa are the Serendipity and the $3 Dip, flies that use a bit of deer hair to help suspend them and/or imitate the emerging midge adult.
We should probably be fishing more midge pupa than we are, particularly in slow moving pools and glides. Midge are hatching all year though most of us notice them a lot more in the Winter and early Spring when not much else is hatching.
If there is a common misconception about midges, it is that they are mosquitoes or biting gnats, which they are not. In fact, the common name for the Chironomidae family is "non-biting midges". Like many aquatic insects, as aerial adults, their main, and for many species, their only purpose is to reproduce and create the next generation. You will often see these harmless little flies around rocks in slackwater areas and in aerial mating swarms - both signs that a midge egg laying event is likely.
Like other aquatic insects, fly anglers are often imitating the adults in two different ways; first as they hatch from pupa to adult and they may take a little time before their are capable of flying away as aerial adults, and second, as they return to mate and lay eggs. Pupa and emerger patterns capture the transition to adulthood. Most midge dry flies imitate a single midge but due to their small size and mating behavior, mating midge clusters are often imitated. The best known of these cluster patterns is the Griffith's Gnat but there are many other options and variations. Cluster patterns allow anglers to fish larger and more visible fly patterns.
Wrapping It Up
Midges are the Rodney Dangerfield of the "Big Four" aquatic insects. Most anglers have stories of a great mayfly, caddis, and/or stonefly hatch they encountered and figured out. Hardly anyone talks about midges the same way - we tend to view fishing them as a necessary evil. Fishing small flies is not for everyone but a well-rounded angler should have some ability to fish these little flies. If nothing else, drop a midge larvae or weighted pupa below your dry fly or nymph in a double rig and you might be surprised how often fish eat #18, #20, and smaller midges.
Or go chase midges on lakes and spring ponds which will change your perspective on midges.
Fly Tying Resources
How to Tie Midge - FLY All SZN (video)
Build a Better Brassie - Clark "Cheech" Pierce / Fly Fish Food (video)